The optical communication system has rapidly grown during the past several years, the global 40 G transmission commercialization process has an obvious acceleration trend, and scale-deployment was started. The acceleration of the commercialization process also spurs the development of industrial chain. Compared with the 10 G transmission system, the 40 G transmission system, under the same physical conditions, requires that the OSNR (Optical Signal Noise Ratio) be increased by four multiples (6 dB), the chromatic dispersion tolerance be decreased by 16 multiples and the PMD (Polarization Mode Dispersion) tolerance be decreased by four multiples, and non-linear effect also becomes more obvious. In order to overcome these limitations to meet the commercial requirements, many measures should be taken, among which modulation and coding technique is the most crucial one. A practical modulation technique is not only limited to the conventional NRZ (No Return Zero) or RZ (Return Zero) binary amplitude shift keying (OOK, on-off key), and many new modulation techniques, for example, DPSK (Differential Phase Shift Keying) modulation, DQPSK (Differential Quadrature Reference Phase Shift Keying) modulation and so on, are used in the optical communication, and especially the DQPSK modulation, which reduces the requirements on the rate, the chromatic dispersion and the polarization mode dispersion of the electrical element, plays an important role in the 40 G optical communication system.
The principle of DQPSK modulation is that because an optical carrier can be expressed as Ei=E exp j[ω0t+φ(t)], where E is the field strength, ω) is the angular frequency of the optical carrier, φ(t) is the modulated phase. DQPSK modulation is to code the information to be transmitted into the differential phase of continuous optical bits. The differential phase is indicated by Δω, which may be a value among [0, π/2, π, 3π/2]. It is assumed that the phase of the k−1th optical bit pulse is θ(k−1). If the subsequent bits are 0, 0, then θ(k)=θ(k−1)+π, if the subsequent bits are 0, 1, then θ(k)=θ(k−1)+π/2; if the subsequent bits are 1, 1, then θ(k)=θ(k−1); if the subsequent bits are 1, 0, then θ(k)=θ(k−1)+3π/2. Of course, the above rule for coding the information to be transmitted with Δφ is not limited to the above mode, for example, it may be that when the subsequent bits are 0, 0, θ(k)=θ(k−1), and if they are 1, 1, then θ(k)=θ(k−1)+π, and so on.
The DQPSK demodulation principle based on the above DQPSK modulation process is that two differential currents are obtained by performing DQPSK demodulation on the received optical signal, these two differential currents carry the modulation phase difference between adjacent optical bits, and the transmitted information is obtained according to the modulation phase difference. FIG. 1 is the block diagram of the existing receiver based on DQPSK modulation. As shown in FIG. 1, the receiver comprises a first demodulator, which is used to perform phase rise and phase cancellation operations on the optical signal transmitted by its two arms and output a first phase rise optical signal and a first phase cancellation optical signal for obtaining the first differential current signal; and a second demodulator, which is used to perform phase rise and phase cancellation operations on the optical signal transmitted by its two arms and output a second phase rise optical signal and a second phase cancellation optical signal for obtaining the second differential current signal. In order to obtain the first differential current signal and the second differential current signal from which the modulation phase difference can be extracted and thereby accurately restore the transmitted information, it is required that the phase difference of the first demodulator must strictly meet the demodulation requirements: the phase difference is π/4, and that the phase difference of the two arms of the second demodulator must strictly meet the demodulation requirements: the phase difference is −π/4, otherwise, additional Optical Signal Noise Ratio price will be introduced.
In order to achieve monitoring and control of whether the phase difference between the two arms in the above demodulator meets the demodulation requirement, a feedback control loop is generally adopted to monitor the phase difference and generate a phase adjusting signal to adjust the phase difference between the two arms in the two modulators so that the phase difference meets the demodulation requirements (locked on the target values of π/4 and −π/4). Currently, the commonly used feedback control method is to implement slight disturbance with a fixed frequency of f and simultaneously monitor that the 2f component in the error signal reaches the extreme value. The intrinsic drawback of this solution is that phase disturbance with a fixed frequency of f will necessarily cause additional Optical Signal Noise Ratio price; the measurement of the extreme value only suggests whether the current phase is equal to the target value, but whether it is greater than or smaller than the target value is unknown; the rate of phase control is limited by the frequency of jitter; and the signal for which the extreme value is measured is in a square relationship with the phase error, and the control accuracy around the target value is relatively low.